In many machine tools, particularly those which involve bending, cutting or forming, it is necessary to provide means for holding the workpiece, or at least portions of the workpiece, stationary with respect to the active or moving element of the tool. For this purpose various types of workpiece supports or holders are secured to work supports or platforms. The position of these work holders, both with respect to each other and to the active element of the tool, have to be adjusted from time to time. This adjustment normally involves a disassembly operation followed by a reassembling of the work holder to the platform. This is a time consuming operation. It is also of the utmost importance that these work holders or pivot block holders, as they may be referred to, be secured to the work platform in such a manner that there is no danger of inadvertent disconnection which would not only be dangerous to personnel but would also, in all probability, render the workpiece scrap. To assure accuracy of the tool, it is necessary that these work holders accurately maintain their position without twisting, bending or shifting in any way. To satisfy the strength, rigidity and safety aspects of these devices, the work holders have been massive in construction and have been attached in a manner such that substantial time and effort has been necessary to attach and detach them from the platform. In general, they have been cumbersome and time consuming whenever it is necessary to change tooling.